He is in the major leagues because he throws with his left arm to a reasonable degree of success. Now in his sixth big league season, he’s compiled a 4.49 ERA and 1.45 WHIP. He can come out of the bullpen and get some outs, but he’s not Mariano Rivera. If he gets into a jam, his manager is going to have to pull him from the game.

So when Morales entered a 3-2 ballgame Wednesday night at Fenway Park and allowed Josh Hamilton, Adrian Beltre and Nelson Cruz to load the bases, it was time for Bobby Valentine to make his way to the mound and call on someone — anyone — to finish the job. In fact, he probably could have used a righty to face Cruz the batter before, but after that walk, it was certainly time for a new arm.

Amazingly, though, Valentine remained in the dugout, leaving Morales in the game to pitch to Craig Gentry. Granted, Gentry did not have a hit on the season before Tuesday night, but it was evident that Morales had nothing to offer on Wednesday night. Evident to everyone except for Valentine, that is.

The manager stuck with Morales, who rewarded him by hitting Gentry in the foot to force in a run.

The score was then 4-2. Valentine had screwed up by letting Morales stay in the game, but he could make amends by taking him out now, with verified Red Sox killer — and right-handed batter — Mike Napoli due up next. Matt Albers, a righty, was warm in the bullpen.

Yet somehow, the hook never came. Morales, who had retired just one of the five batters he had faced to that point, stayed in the game. And it only took three pitches for Napoli to blow the game wide open with a two-run double to give the Rangers a 6-2 lead.

Out of the dugout, finally, walked Valentine, met with a resounding chorus of boos from the fans at Fenway … or at least those who hadn’t decided to bolt for the exits upon the sight of Morales being left to get shelled.

“I wanted to keep confidence. I wasn’t gonna pull the plug too soon,” Valentine explained after the Sox’ eventual 6-3 loss to Texas. “He’s had four good outings for us. I was hoping to salvage that one. It didn’t work.”

No, it did not, and it turned Valentine’s week from awful to impossibly worse. Of course, that doesn’t make Valentine an entirely sympathetic figure, as he’s largely responsible for much of the backlash he’s received in Boston in the past week.

It began Friday, as Fenway Park opened its 100th birthday celebration. The Opening Day ceremonies were full of pomp and circumstance, as well as boos for Bobby V. The man who had steered the team to a 1-5 start was introduced for the very first time at Fenway as Red Sox manager with disapproval from the Fenway crowd. He was going to have to win them over.

He and his team spent the weekend doing exactly that, beating Tampa three times by a combined score of 31-11. The ship was righted. Until Bobby V spoke.

Unprompted, Valentine discussed his All-Star infielder and fan favorite, Kevin Youkilis, telling a local TV station that the veteran was not “as physically or emotionally into the game as he has been in the past for some reason.” Anything at all said in Boston turns into a story, but outspoken criticism of a player’s effort or focus? Such comments invite hysteria, and that’s what filled Fenway on Monday morning.

With the Sox in the ballpark early for their annual Marathon Monday matinee, the response was quick. Youkilis said he was confused by such a concept, while Dustin Pedroia, when asked if he thought Valentine was trying to motivate Youkilis, was not having any of it.

“That’s not the way we go about our stuff here,” Pedroia asserted. “Maybe that works in Japan.”

Ask the fans to pick a side between their beloved, gritty second baseman who’s won Gold Gloves, an MVP and a World Series, or the new manager who’s been around just a couple of weeks. You won’t be surprised by their choice.

Later that afternoon, it was Valentine’s on-field decision-making that left folks scratching their heads. With Daniel Bard on the mound for just his second career start after spending the past four professional seasons as a reliever, pitch count should have been a priority. Bard was at 103 pitches in the seventh, after surrendering a two-out walk to Sean Rodriguez and a single to Desmond Jennings. Bard needed to come out of the game. He was cooked. The most pitches he had thrown in one outing in the entire 2011 season was 32, and this was just his second start of the year.

Yet Valentine inexplicably left him in the game. Bard walked Carlos Pena on just four pitches, failing to even come close to the strike zone with any of them. Valentine had screwed up by leaving Bard in, but still, just like he did on Thursday with Morales, he had a chance to limit the damage. And just like Thursday, he chose not to.

Valentine left Bard in to face Evan Longoria. Ball one, ball two, ball three, ball four. Rodriguez jogged home to score the game’s first run. The Rays would win 1-0.

In just three days, Valentine’s decisions — or lack thereof — have directly turned a 0-0 game into a 1-0 deficit and a 4-2 game into a 6-2 hole. It’s truly difficult for a manager to lose two out of three games almost entirely by himself, but it’s just as hard to say Valentine didn’t do that this week.

As Valentine walked to the dugout Thursday night after finally taking out Morales (Matt Albers would do what he should have done several batters earlier by inducing an inning-ending double play), Valentine was shown no mercy from the fans remaining at Fenway.

“Go back to Japan!”

“No one wants you here!”

“Go back to the Mets, you bum!”

Even Valentine wasn’t much a fan of himself on Wednesday.

“I was booing myself,” he said of the reaction.

It might not have been the warm reception he was expecting from the folks that fill “Friendly Fenway,” but it’s clear through his first week at home that like many managers before him, he’s going to have to earn respect from this city before receiving praise.

That’s going to be tough, too, as his first full week of work at Fenway will come to a close Friday, when the Red Sox officially celebrate the 100th anniversary of the ballpark. Every living player and manager who wore the uniform was invited back to the park, a list that includes Terry Francona, by many measures the greatest manager the team has ever had.

Francona will assuredly be greeted by a rousing standing ovation from a fan base that hasn’t yet gotten the proper forum to thank him for his eight years and two championships.